Messaging (`lv_msg`) is a classic []publisher subscriber](https://en.wikipedia.org/wiki/Publish%E2%80%93subscribe_pattern) implementation for LVGL.
## IDs
Both the publishers and the subscribers needs to know the message identifiers.
In `lv_msg` these are simple `uint32_t` integers. For example:
```c
#define MSG_DOOR_OPENED 1
#define MSG_DOOR_CLOSED 2
#define MSG_USER_NAME_CHANGED 100
#define MSG_USER_AVATAR_CHANGED 101
```
You can orgnaize the message IDs as you wish.
Both parties also need to know about the format of teh payload. E.g. in the above example
`MSG_DOOR_OPENED` and `MSG_DOOR_CLOSED` has no payload but `MSG_USER_NAME_CHANGED` can have a `const char *` payload containing the user name, and `MSG_USER_AVATAR_CHANGED` a `const void *` image source with the new avatar image.
## Send message
Messages can be sent with `lv_msg_send(msg_id, payload)`. E.g.
```c
lv_msg_send(MSG_USER_DOOR_OPENED, NULL);
lv_msg_send(MSG_USER_NAME_CHANGED, "John Smith");
```
## Subscribe to a message
`lv_msg_subscribe(msg_id, callback, user_data)` can be used to subscribe to message.
The callback should look like this:
```c
static void user_name_subscriber_cb(void * s, lv_msg_t * m)
{
/*s: a subscriber obeject, can be used to unscubscribe*/
/*m: a message object with the msg_id, payload, and user_data (set durung subscription)*/
...do something...
}
```
From `lv_msg_t` the followings can be used to get some data:
-`lv_msg_get_id(m)`
-`lv_msg_get_payload(m)`
-`lv_msg_get_user_data(m)`
## Subscribe with an lv_obj
It's quite typical that an LVGL widget is interested in some messages.
To make it simpler `lv_msg_subsribe_obj(msg_id, obj, user_data)` can be used.
If a new message is published with `msg_id` an `LV_EVENT_MSG_RECEIVED` event will be sent to the object.
